Generally, a worm deceleration mechanism is used for the deceleration apparatus in an electric motor equipped with deceleration device that operates the power window system of a vehicle. An electric motor equipped with deceleration device in which this worm deceleration mechanism is used has a structure in which a substantially rod-shaped electric motor is mounted from the outside on a side wall of a substantially cylinder-shaped deceleration device that contains a worm wheel inside it (see, for example, Patent document 1).